Partly-hydrolyzed cellulose (also referred to in the literature as cellulose crystallites or nanocrystalline cellulose) is most commonly produced by the controlled acid hydrolysis of cellulose using sulfuric acid. The cellulose may be from various sources, including wood pulp and cotton. The less dense amorphous regions along the cellulose microfibril are more susceptible to acid attack during hydrolysis and cleave to give the partly-hydrolyzed cellulose. Acids other than sulfuric, such as hydrochloric acid, phosphoric acid, etc., or mixtures of acids, may be used. For simplicity of explanation, the following discussion focuses on the use of sulfuric acid and the removal of sulfate.
At high ionic strengths, such as that caused by the residual sulfuric acid from hydrolysis, the individual nano-particles of crystalline cellulose tend to agglomerate together into larger particles which settle and concentrate under gravity, or in a centrifuge. To obtain the desired discrete nano-particles, the ionic strength of the solution must be reduced to very low levels. To achieve this desired purification, one or more steps of dilution followed by concentration are typically employed to separate the bulk of the sulfuric acid (about 90%) from the partly-hydrolyzed cellulose; however, as the acid concentration and associated ionic strength decrease, the partly-hydrolyzed cellulose particles deagglomerate into smaller aggregates to a point where they no longer separate efficiently by gravity or centrifugation and alternative methods must be employed to further reduce the acid concentration.
To achieve the desired final purity of the partly-hydrolyzed cellulose suspension, a very fine filter which retains the partly-hydrolyzed cellulose particles combined with continuous or step-wise water washing is typically employed. This washing step is often referred to in the literature as dialysis or diafiltration, it typically employs an ultrafiltration membrane-based filter with a retention of about 200,000 Daltons or less. Although a significant portion of the starting acid has usually been removed previously as discussed above, concentrations must be further reduced by several orders of magnitude so that acid does not affect end-use properties of the cellulose product. Even with a highly efficient dialysis system, very large filtration surface areas and flow rates or dilution times are required relative to the quantity of partly-hydrolyzed cellulose produced.
While generally improving overall viability, decantation or centrifugation is not strictly required as an initial purification step. The desired final purity can be achieved using dialysis alone; however, this increases dialysis washing requirements and acid recovery costs significantly.
The above process steps which have been employed to produce partly-hydrolyzed cellulose are described in: (1) U.S. Pat. No. 5,629,055 to Revol et al.; (2). Jean-François Revol, Louis Godbout, Xue-Min Dong, Derek G. Gray, Henri Chanzy, and Georg Maret, “Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation,” Liquid Crystals, (1994) Vol, 16, No. 1: 127; and (3) Xue Min Dong, Tsunehisa Kimura, Jean-François Revol, and Derek G. Gray, “Effects of Ionic Strength on the Isotropic-Chiral Nematic Phase Transition of Suspensions of Cellulose Crystallites,” Langmuir, (1996) Vol. 12: 2076. U.S. Pat. No. 5,629,055 also discloses the use of a mixed bed ion exchange system as a polishing step following dialysis. This is further discussed in (4) Tiffany Abitbol, Elisabeth Kloser, Derek G. Gray, “Estimation of the surface sulfur content of cellulose nanocrystals prepared by sulfuric acid hydrolysis,” Cellulose, (2013) 20; 785-794.
The prior art dialysis purification process described above also removes sugars produced in hydrolysis as well as other soluble impurities; however, these are typically present in lower quantities than the sulfuric acid such that their removal is not normally the limiting factor in the purification of partly-hydrolyzed cellulose.